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It is called the lateral shift.

Suppose the light rays are incident on medium2 (refractive index μ₂) from medium1 (refractive index μ₁) at angle of incidence i. Let the angle of refraction be r.

Then μ₁ sin i = μ₂ sin r.

Now the angle of incidence at the second surface S₂ (parallel to surface S₁) is r and the angle of emergence from medium 2 will be equal to i itself. So it is parallel to the incident ray in medium 1.

But the two rays in medium 1 are parallel but distanced by lateral shift = L.

   L = t Sin (i-r) / Cos r                 where t = thickness of the medium 2.

2 3 2